Tuesday, 4 September 2012

KRAKATAU

KRAKATAU

Indonesia contains over 130 active volcanoes,
more than any other country on
earth. They comprise the axis of the Indonesian island arc system, which is
generated by northeastward subduction of the Indo-Australian plate. The great
majority of these volcanoes lie along the topographic crest of the arc's two
largest islands - Java and Sumatra. The islands are separated by the Sunda Straits, which is located
at a distinct bend in axis of the island arc volcanoes, from a nearly east-west
orientation in Java to a northwest-southeast orientation in Sumatra. Krakatau
is one of a several volcanic islands in the Sunda Straits located above an
active north-northeast trending fault zone, an orientation quite distinct from
the main island-arc trend. Although relatively small when compared to the
largest volcanoes along the island arc, Krakatau and its associated volcanoes
have shown the capacity to generate highly explosive eruptions.

ENORMOUS SEA WAVES

The cataclysmic
blasts of August 27 generated mountainous tsunamis, up to 40 m tall, that
ravaged coastlines across the Sunda Straits. Many of the closest islands were
completely submerged. After first being overwhelmed by massive pyroclastic
flows (see below), Sebesi Island northeast of Krakatau, was innudated by
mammoth sea waves. These tsunami stripped away all vegetation, washed ~3000
people out to sea, and destroyed all signs of human occupation. Although
located at seemingly safe distance, 80 km east of the Sunda Straits, the
low-lying Thousand Islands were buried by at least 2 m of seawater and their
inhabitants had to save themselves by climbing trees.

Eyewitness accounts
of the massive waves came from passengers of the Loudon, who survived the barrage only through the heroic efforts
of its Captain Lindemann. The ship was anchored in Lampong Bay, near the
village of Telok Betong when the first of several waves arrived on Monday
morning:

Tsunamis were clearly responsible for most of
the fatalities at Krakatau. However, ~4,500 deaths (over 10% of the total) have
been attributed to falling tephra and hot pyroclastic flows. The amount of
tephra generated is thought to be about 20 cubic kilometers, or twenty times
that of the destructive Mt. St. Helens eruption in 1980. Near Sumatra, the
Sunda Straits were clogged with so much debris that it looked like solid
ground. Relief ships were unable to reach coastal communities like Telok Betong
for weeks. Over the ensuing months, storms and high-tides would disperse thick
banks of floating pumice beyond the Straits, into the Java Sea and Indian
Ocean. Ships thousands of kilometers from Krakatau would report huge fields of
this floating debris for months after the eruption. One such accumulation
floated 8,170 km, until it reached Durban, South Aftica in September, 1884.

About 2000 of the corpses in southern Sumatra
had severe burns, indicating that they had been scorched to death, peresumably
from pyroclastic flows. Although the behavior of pyroclastic flows and surges
over water is poorly contrained by direct observations, the evidence suggests
that they can travel great distances over open water. One compeling feature of
the Krakatau eruption is that the pyroclastic flows appear to have travelled an
incredible 40 km across the Sunda Straits, where they remained hot enough to
cause the burn-related fatalities on Sumatra. These same flows, however, were
also recorded by several ships located at greater distances. On August 27, the Louden (see above) was located ~65 km
north-northeast of Krakatau when it was struck by severe winds and tephra, and
the W.H. Besse was located at
~80 km east-northeast of Krakatau when it was hit by hurricane-force winds,
heavy tephra, and the strong smell of sulfur. At these greater distances, the
pyroclastic flows were at lower temperatures so that the ships and crew
survived.

How is it possible
for pyroclastic flows to travel such great distances?Pyroclastic flows are hot mixtures of solid
particles and expanding volcanic gases. While advancing over water, the base of
the flow will conert the water to steam. The rapid expansion of water to vapor
greatly enhances flow fluidization and inhibits the deposition of particles,
particularly the low-density pumiceous particles, thus allowing the flow to
travel tens of kilometers over flat oceanic waters. This mobility was first
recognized during the 1902 eruption of a pyroclastic flow from Mt. Pelée, which destroyed the coastal city of St.
Pierre, only to continue across open waters to overturn and burn ships anchored
several kilometers offshore.

After travelling 40 km over the Sunda
Straits, pyroclastic flows struck southern Sumatra with a vengence, remaining
hot enough to incinerate entire villages and burn all vegetation before loosing
impetus on the highly forested mountainsides. The wife of Controller Beyerinck
from the Sumatra village of Ketimbang described her expereince on the morning
of August 27, when the outermost edges of a pyroclastic flow enveloped her
family and their acquantainces, killing some and sparing others.

ATMOSPHERIC
EFFECTS

Tephra from the eruption fell as far as 2,500
km downwind in the days following the eruption. However, the finest fragments
were propelled high into the stratosphere, spreading outward as a broad cloud
across the entire equatorial belt in only two weeks. These particles would
remain suspended in the atmosphere for years, propogating farther to the north
and south before finally dissipating.

The stratospheric cloud of dust also
contained large volumes of sulfur dioxide gas emitted from Krakatau. These gas molecules
rapidly combined with water vapor to generate sulfuric acid droplets in the
high atmosphere. The resulting veil of acidic areosols and volcanic dust
provided an atmospheric shield capable of reflected enough sunlight to cause
global temperatures to drop by several degrees. This aerosol-rich veil also
generated spectacular optical effects over 70% of the earth's surface. For
several years after the 1883 eruption, the earth experienced exotic colors in
the sky, halos around the sun and moon, and a spectacular array of anomalous
sunsets and sunrises. Artists were fascinated by these aerial displays and
captured them on canvas. The painting shown here is one such sunset captured by
the artitst William Ascroft on the banks of the River Thames in London, on
November 26, 1883 (Courtesy of Peter
Francis).

According to SimonWinchester, despitewhat happensin
the life ofKrakatauwhichused to be veryscaryrealitiesof geological, seismicandtectonicin Java andSumatra,the strangewill ensurethatwhat used tohappenat some pointwillhappenagain. No oneknowsexactly whenthe volcanowillerupt.
Somegeologistspredict
eruptionswill occurbetween2015-2083. However,the effectof theearthquake
on theIndian OceanonDecember 26, 2004also can not beignoredprevious eruptions.

According to ProfessorUedaNakayamavulcanologistoneJapanese national,Anak Krakatauis stillan activeandrelatively safealthoughthere is oftena small explosion, there are onlycertaintimes
whenthe touristsare prohibited
from approachingthe areabecause
of the dangerthatspewedlavavolcano. Otherexpertssaid there was noplausible theoryofAnak
Krakatauwill bere-erupted.
Even if there areat least 3ormorecenturiesafter2325ADBut clearly, the numberof victims affectedmore powerfulthanthe previouseruptions.
Anak Krakatauis nowby the general publicbetter known as"Krakatoa" also, though themountainis actuallygrowing
newpostprevious eruptions